This invention relates to aromatic homopolyimide and copolyimide films derived from an aromatic dianhydride and a chlorinated aromatic diamine having low water absorption, high elastic modulus and elongation and which can be heat-processed without deleterious decomposition.
Aromatic polyimides and copolyimides are well-known in the art to have outstanding heat resistance and tensile properties making them commercially useful as molded products such as films or fibers. However, the water absorption of such polyimide films is undesirably quite high. High water absorption affects deleteriously the dimensional and the thermal stability of the film, reduces mechanical property values and limits fabrication latitude. Furthermore, the introduction of stiff copolymer segments into the polyimide polymer chain causes the polymer film to become rigid and disadvantageously brittle during heat-processing.
It has now been found that the introduction of specific chlorinated aromatic diamines into the aromatic (co)polyimides provides films having water absorptions considerably lower than the corresponding non-chlorinated (co)polyimide films. Furthermore, when preparing the (co)polyimide films by a chemical conversion process using an anhydride dehydrating agent and a tertiary amine catalyst, the intermediate films are very flexible and can be advantageously heat-processed without embrittlement. Quite surprisingly, the chlorinated (co)polyimides also provide films having a much higher elastic modulus and elasticity than the corresponding non-chlorinated (co)polyimide films making useful as base films for magnetic recording tapes and as substrates for flexible printed circuits and tape-automated bonding.
U.S. Pat. No. 4,530,993, issued to Jinda et al. on Jul. 23, 1985, discloses a polyimide film derived from pyromellitic dianhydride and 2,2'-dichlorobenzidine having a high tensile strength and elastic modulus and a high heat resistance. The invention (co)polyimide films do not contain 2,2'-dichlorobenzidine as a component.
Japanese patent publication 62-79227, published on Apr. 11, 1987, discloses aromatic copolyimide films derived from pyromellitic dianhydride and mixtures of 2,2'-dichlorobenzidine with benzidine, 3,3'-dimethylbenzidine and 2-chlorobenzidine having good mechanical properties and high heat resistance. The invention (co)polyimide films do not contain dichlorobenzidine and dimethylbenzidine as diamine components.
U.S. Pat. No. 4,690,999, issued to Numata et al. on Sep. 1, 1987, discloses a polyimide having a low thermal expansion coefficient comprising at least one aromatic ring as a structural component which can rotate around its molecular axis but has no flexibility in another direction and which is oriented in at least a uniaxial direction. The polyimides are derived from various aromatic dianhydrides and aromatic diamines with the imide rings bonded to the aromatic phenyl rings at the para-positions to provide rigid, crystallized structures. There is no specific disclosure, however, of using chlorinated aromatic diamines as a component in these rigid structures.